Interpretive Summary: Arbuscular mycorrhizal [AM] fungi are naturally occurring soil fungi that infect the roots of crop plants and assist them in the uptake of nutrients from the soil. They also provide the plant some protection against disease. Research has shown that inoculating plants with these fungi prior to planting them in the field, as in vegetable production, increases the yield. Inoculum of these fungi is currently commercially available, but quantities needed for large-scale plantings can be expensive. We have developed a method whereby farmers can produce inoculum of these fungi on their farms, saving the associated processing and shipping costs. Grass seedlings, pre-infected with AM fungi, are planted into enclosures filled to a depth of 20 cm with a mixture of compost and vermiculite. The enclosures are weeded and watered throughout the summer, during which the fungi proliferate as the roots grow throughout the mixture. The mixture is ready for use as inoculum the following spring. Upwards of 100 million propagules of these fungi have been produced in a section of an enclosure only three fourths of a meter square, enough to inoculate one million vegetable seedlings.

Technical Abstract:
On-farm production of arbuscular mycorrhizal [AM] fungus inoculum can make the benefits to crop growth and yield of inoculation with AM fungi available to more farmers by reducing costs. Paspalum notatum Flugge seedlings, pre-colonized by one of six AM fungi, were transplanted into raised bed enclosures. Media within the enclosures was vermiculite mixed with either field soil or yard clippings compost in year 1 and vermiculite mixed with yard clippings compost or dairy manure/leaf compost in year 2. Compost and vermiculite mixtures out-performed soil based mixtures in year 1. Growth of pre-inoculated plants in a 1:4 [v/v] mixture of yard clippings compost and vermiculite produced more inoculum than growth in a 1:99 [v/v] mixture. The effect of supplemental nutrient addition was studied in the second year. Water, inorganic nutrient solution minus P, or fish protein digest were added to inoculum production enclosures containing either yard clippings compost: vermiculite [1:4, v/v] and dairy manure/leaf compost: vermiculite [1:12, v/v]. Results indicated that supplemental nutrient addition was unnecessary. Up to 912 propagules cm-3 were produced in the yard clippings compost: vermiculite mixtures. Examination of spores in the final inoculum indicated that not only were the introduced fungi present, but that AM fungi present in soil introduced to the compost during turning the previous year had been propagated. Therefore, this method may be used to produce inoculum of indigenous, locally adapted isolates of AM fungi.